Field of the Invention
The present invention relates to an image pickup apparatus and a method of controlling the image pickup apparatus.
Description of the Related Art
Hitherto, there has been proposed an actuator in which, through the generation of elliptic motion at a predetermined position of a vibrator, the vibrator and a friction member that is brought into friction contact with the vibrator are relatively displaced. Further, an image pickup apparatus including such actuator as a drive source for a mechanism portion of a camera and a lens has also been proposed.
The vibrator forming the above-mentioned actuator is constituted, for example, such that a piezoelectric element serving as an electromechanical energy conversion element is connected to an elastic body such as a metal, and two-phase AC voltages having different phases can be applied to the piezoelectric element. A vibration wave is excited on a surface of the vibrator through application of the voltages to the piezoelectric element to pressurize the vibrator to the friction member, to thereby relatively displace the vibrator and the friction member. In this manner, the vibrator is slid on a surface of the friction member with desired drive force.
The actuator has a feature of being able to be held at an original position even after the application of the voltages is stopped. The reason for this is as follows. The vibrator is in a pressure-contact with the surface of the friction member, and hence friction force is generated. Such friction force is also referred to as holding force.
However, when the vibrator is held at the same position on the surface of the friction member for a long period of time, a slight amount of water in air aggregates in a contact portion between the vibrator and the friction member, with the result that friction force, that is, holding force decreases. When external force is applied under a state in which holding force is decreased, the actuator may move in some cases. When the actuator configured to drive a lens moves, an out-of-focus problem may occur.
In view of the foregoing, a technology of recovering decreased holding force has also been proposed (Japanese Patent No. 5110824).